Amplifier tube arrangement



2 Sheets-Sheet l TTORNEY.

Feb. I511, 1941- H. ,ROTHE ETAL AMPLIFIER TUBE ARRANGEMENT Filed April2, 1938 I. m N m A l U r I2 2 M U U I h 2 1 I a, a 2 o I! A P 0 N t\ 0 21 I I M c i U .M F U, a v v N m A l M Q.

Feb. 11, 1941. H. ROTHE ErAL 2,231,372

AMPLIFIER TUBE ARRANGEMENT Filed April 2, 1958 2 Sheets-Sheet 2 HACgkANT.

RNER KLEEN BY M ATTORNEY.

Patented Feb. 11, 1941 2,231,372 AMPLIFIER TUBE ARRANGEMENT Horst Rothe,Berlin-Charlottenburg, and Werner Kleen, Berlin-Wilmersdorf, Germany,assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b.H., Berlin, Germany, a corporation of Germany Application April 2, 1938,

In Germany April 3,

2 Claims.

This invention relates to electron tubes and more particularly to anovel method of, and

means for, the creation and technical utilization of negative inputcapacitances of vacuum tubes.

It is a well known fact that between the various electrodes confined inpacitances are set up a discharge tube ca,-

, and that the value of the control grid to filament capacitance, andalso of the filament to plate as a function of the capacitance may bevaried working slope to which the tube has been adjusted. The inventionis concerned with the fact,

both by calculus as well that it is possible to in the presence ofprovable and demonstrable as by actual experiments,

insure in a discharge tube,

suitable discharge conditions,

also negative capacity values which are utilizable in practice in widelydifferent ways.

Suppose a tube comprises a grid, and an anode.

constitute the coats of of which, in cold state of the cathode (coldshall be designated by Cgk.

capacitance) soon as the cathode cathode, a control The grid and thecathode a condenser the capacity As is heated and emits electrons, thedischarge path becomes filled with electrons, and by virtue occasioncharges upon effective or active capacitance a result, a different, willarise between the of influence actions, these the condenser coats. As

grid and the cathode. Ap-

plying across grid and cathode an alternating potential of angularvelocity w, there will flo-w between these two electrodes an alternatingcurrent which, in the presence of a negatively biased grid, comprisestwo components, that is to say (1) the capacitive displacement currentwhich flows through the static capacity Cgk, and (2) the influencecurrent.

The latter is due to the fact that the intensity of the ing over to theanode electronic current pass- (convection current) is acted upon by thecontrol action of the grid, and

that it causes upon the grid a charge which is fluctuating in the sameway. The influence current as a general rule comprises a reactive and anactive component. In other words, the input impedance of the tube is acomplex value, and it is representable by an ohmic resistance and acapacity in parallel. In this connection the value of this capacityshall of exclusive interest be studied. In fact, what is her-e is thedeparture AC k from the static capacity value Cgk previously referredto.

For ACgr the slope at the adjusted input portance. Assuming ideal shape,that is of capacitance Acgk a of the tube characteristic point is ofdecisive imfor the characteristic the It=l :U then the increase long thewhole characteristic is positive and constant. of U, also hereinafter3/2, no fundamental Serial No. 199,604

referred to as 1L,

such characteristic is representable by the position of variouscharacteristics with exponent 3/2. However, the situa the case ofcharacteris having an exponent sma the relationship between exponent n,it will be dis increment initially stay smaller values as the value of ndecrease 3/2 towards zero.

where the exponent is'unity (11:1), or

the slope of the characteristic be again with growing emission cu versesin sign and changes from through zero to negative values.

positive gins to decrease rrent, Acgk re- If the exponent exceeds hangeoccurs since each supertion becomes different in tics or portionsthereof ller than 3/2. Tracing ACgk and the value covered that thecapacity 5 positive, but assumes s from Approximately at the point wherey D values The influence current flowing to the grid upon application ofan alternating voltage is then inductive.

negative crest and returns to finally attains a uration range of acharacteristic.

The novel features which we believe our invention are set forth incharacteristic of particularity in the appended claims; the

and method of stood by reference to taken in connection with zero for71:0, and this corresponds to the satto be invention itself, however, asto both its organization operation will best be underthe following descthe drawings in we have indicated diagrammatically some organizationswhereby our invention may be carried into effect.

In the drawings: Figs. 1 and 2 are grap potential relations in a vacuumtube,

Fig. 3 is a schematic circuit diagram for producing frequencymodulation,

Fig. 4 graphically capacitance characteris Fig. 5 schemati of anintermediate frequency ing the invention,

Fig. 6 is a circ lustrating a modi the invention.

The physical condi little more fully Referring to Fig. 1, I and of theseat least electrode I is and by reference to Figs. 1

grid-s ription which circuit hical representations of represents thegrid-plate tic over the grid voltage, cally shows a circuit diagramamplifier embodyuit diagram schematically ilfied arrangement embodyingtionsshall be discussed a and 2.

and II denote two electrodes,

haped.

At the same time there are plotted the potentials prevailing atdifferent pl space, that is, also the pote very electrodesi O designatesa source 0 aces of the discharge ntials obtaining at the f electrons(real or virtual cathode) whence a stream of electrons I fiows throughthe grid I into the discharge space between electrodes I and II.Electrode I is subject to the constant, positive potential U1. Thepotential U2 of the electrode II shall also be positive, thoughvariable. If, first, U1=U2 there is produced between the electrodes Iand II a distribution of ing to the dash-line graph I. In the spacepreceding the electrode II prevails a positive, that is electronaccelerating, field intensity 6. If, then, the potential U2 is reduced,at a very definite voltage U2 the field intensity in front of theelectrode II will just be equal torzero (graph 2) As the potential ofelectrode II is diminished still further, say, to U2", there arises adistribution of the potential in accordance with graph 3, that is tosay, the field intensity becomes positive again. Now, since for thecharge Q upon the surface of the electrode II there holds this relation:

it follows that the charge upon the electrode II caused by electrons inthe space, as a function of U2, corresponds to the graph shown in Fig.2. It is to be noted, however, that this curve is to give merely aqualitative View, for the accurate calculation is rather complicated.

The capacitance of electrode II in reference to electrode I is:

in other words, equal to the differential quotient of the graph in Fig.2. For U2 U2' therefore, the capacitance is positive, and for U2=U2'i0and for U2 U2' it is negative. stricter calculation shows that, from apurely theoretical viewpoint, in the presence of suitable conditions,the capacity may become negative and infinitely high.

If, then at the place of the electrode II there is mounted a negativelybiased control grid having a positive electrode (screen grid, plate)disposed to the rear thereof, it will be seen that the sameconsiderations hold good for the control grid. In other words, thecapacitance between electrode I and the control grid may decrease to thezero level and may even become negative. However, the same rule appliesalso for the capacitance between the control grid and the positiveelectrode disposed to the rear thereof so that, in the presence ofconvenient operating conditions, the total input capacity of a controlgrid may drop down to zero or may even become negative.

It has already been pointed out above that the influence currentcontains a reactive and an active component. While it is true that thesize and the sign of both components are a function of the electrontransit time, actual measurements made on the 'I'elefunken tube SP1, forinstance, have demonstrated that the change in capacity Acgk for thecustomary electrode systems, down to wave-lengths of around 5 meters, isindependent of the frequency, and that there occurs an inappreciabledecrease only for still shorter waves.

Now, in order that in the light of what precedes, a negative capacitymay be obtained be tween the grid and the filament of an electronictube, it will be necessary and also sufiicient to ascertain by suitabledischarge conditions and more particularly by convenient choice of thegrid biasing voltage, such a range of the plate currentgrid voltagecharacteristic (mutual conductance) that it is representable by anexponential funcpotential correspondto generate oscillations;

tion with an exponent 12 less than unity (n 1). This feature is present,for instance, in all tubes with saturation properties, regardless ofwhether true saturation due to limited emissivity of the filament ispresent or merely the so-calledv pseudo saturation occasioned byspace-charge phenomena. To the first group belong all tubes withmetallic cathodes (tungsten filaments), with thoriated cathodes, ordistillation cathodes, that is, ox-

ide cathodes presenting a smooth surface. In the second group are foundthe tubes predicated for their operation upon current-distributioncontrol as well as space charge grid type of tubes. The characteristicsof the tubes based upon currentdistribution control, as long as thecurrent densities are low and as long as space-charge effects are notappreciably present, that is to say, especially in a state ofdissolution of the virtual cathode, obey an exponential function with anexponent n: and it is for this reason that they prove particularlyappropriate for producing a negative capacity. It is also expedient thatthe grid characterized by the negative capacity arising between it andthe filament be decoupled from the anode which is at an alternatingpotential by the aid of a screen grid maintained at a constantpotential, with a view to precluding all chances of the alternatingplate potential reacting upon the influence process, and to creatingconditions so that a state exists in which calculation is easy and wherereaction is absent.

The negative capacity could readily amount to a few micro-micro-faradsand may thus exceed the cold capacity between the pair of electrodesreferred to. This holds good particularly Where this capacity is set upat a third or a succeeding grid in a tube operating withourrent-distribution control because the influence actions are thenparticularly marked.

Inside the same range as the negative capacity there arises in mostinstances also a negative active component of the input impedance. Thismay be used either for generation of oscillations or for the partialregeneration of an associated circuit; or by choosing suitabledimensions and sufficiently high damping of the associated circuits,conditions could also be so made that no self-oscillating will happen,but that stable conditions are established. Since the negative realcomponent of the input conductance, upon increase of the frequency, actsmore and more de-attenuating (regenerating), it is thus possible toobtain an increase in plate resistance in the preceding stage growingwith the frequency; and this insures an increase in the gain for thehigher frequencies which is desirable in a great many instances.

So far asthe utilization of the negative capacity is concerned, which isobtainable by ways and means as hereinbefore suggested, there are manypossibilities to accomplish this. Fig. 3 shows a circuit organizationadapted to frequency modulation. Tube R contains a saturable cathode K,e. g. a thoriated cathode, a grid G and an anode A. Between the grid andthe filament is connected the resonant circuit LC which is tuned to thecarrier wave. Upon the grid G is impressed from the source of directcurrent Voltage supply Ug such a biasing potential that the Workingpoint comes to lie, inside the region of the characteristic in whichboth the input resistance Rg as well as ACgk assume negative values. Theresult is that the circuit LC is caused the latter are amplified by thetube so that across the plate-circuit impedance La amplified alternatingpotentials are set up which are taken off across the terminals a, b, andimpressed upon additional amplified stages or a load resistance(antenna). If, then, the grid voltage Ug is caused to vary at the rhythmor rate of modulation, or if in series with the stabilized potential Ug,by the aid of a transformer T, a modulation potential is introduced,then Acgk, that is, acapacity in parallel relation to C, will vary andthereby the frequency of the generated oscillations will also vary. Ifthe modulation potential is prevented from exceeding the range in whichthe grid resistance is negative and capable of causing oscillations inthe associated oscillatory circuit LC, there results steadygeneration ofa frequency-modulated oscillation.

Another useful field of application of negative capacity is inconnectionwith short-wave cirof all kinds in which the inthe tube hasheretofore been cuit organizations put capacitance of i very annoying.Inasmuch as it is readily possible to obtain negative capacitances, thesize of the cold capacity, between the input electrodes of a tube,adjustments could be made, for instance, in such a way that the coldcapacitance will just be neutralized by the negative capacity. Thenatural frequency of a frequencygoverning grid circuit will thus not bealtered by inserting the tube. It is therefore possible also to permit afrequency standard, say, a crystal oscillator or a feebly damped circuitto operate actually at its real natural frequency, without it beingnecessary to dispense with the fixed cou pling of this circuit as in thepast. It is moreover feasible to neutralize base capacitances also bythe negative capacity. It is thus permissible to manufacture short-wavetubes with a base which was prohibitive in many instances in the earlierart. The base losses may be minimized by the use of a suitable ceramicmaterial to a point where they are actually negligible.

If the negative capacity exceeds the cold capacitance of the inputelectrodes in magnitude, it becomes possible to neutralize part of thecapacity of the input circuit. In short-wave circuit organizations theinput circuit consists, for instance, only of an inductance coil or turnwhose natural period is governed by the distributed capacity of thewinding (coil) and the leads. It has heretofore been impossible in theart to reduce the frequency of the oscillations generated in a circuitorganization below the natural frequency of the externally associatedoscillatory structure. However, by the aid of the negative capacity thishas here become feasible; as a result the lower frequency limit of wavegeneration may be reduced. It is immaterial in this connection whetherthe excitation of the oscillatory system is accomplished by the negativeinput impedance of the tube or by application of such means as arewellknown in the earlier art, for instance, feed-back or regeneration. etc.

Another chance to utilize the variability of the capacitance between theinput electrodes of a tube exists in connection with the so-called sharptuning of a receiver apparatusor of a wave generator. In modern receiversets possessing a high degree of sensitiveness and high selectance it isvery important that the set be precisely tuned to the carrier of thestation to be received lest part of the one side-band be cut off orportions of an adjoining side-band be passed. Suggestions have been madein the art to the end of relieving the listener of the work of effectingexact tuning to the desired carrier wave. In fact, all the listener iscalled upon to do in such a case is to set the apparatus coarsely, whilethe apparatus will automatically adjust itself to the middle of thefrequency band to be received. Such automatic tuning, however, in thepast required mechanical drive mechanisms for the tuning means, and thismade the scheme rather complicated in construction.. Now, the use ofnegative capacity according to the invention introduces an essentialsimplification and great convenience.

In Fig. 4 the capacity variation Acgk has been plotted as a function ofthe grid biasing voltage 'Ug. It will be seen that the capacityvariation changes from a positive maximum to a negative crest value andthereafter decreases again towards zero. The region bounded by thepoints p, q, r, is to be used for sharp tuning of a circuit, thequiescent (neutral) point being at r.

Fig. 5 shows schematically an exemplified embodiment of a receiverdesigned for precise tuning according to the invention. The oscillationspicked up by the antenna ANT are fed into the input circuit of a mixerstage M in which combination with the oscillations furnished from aheterodyne U is effected, the frequency of the latter being governed bya resonance circuit LC. The latter shall be assumed to be includedbetween the grid and the cathode of a tube whose capacity variation ACgkcorresponds to the curve shown in Fig. 4. It is immaterial in thisconnection whether the wave generation is also produced by this tube orwhether the tube is solely provided for the production of the variablecapacitance. The beat frequency formed in the mixer tube is fed to an I.F. amplifier Zw which comprises sharply tuned (highly selective)band-pass filter circuits. The I. F. is demodulated in a succeedingrectifier D as known in the art and then amplified in an audio frequencyamplifier NF to which the loudspeaker Lsp is connected.

In coupling relation with the output of the I. F. amplifier are tworesonance circuits Km and KM, one of these being fixedly tuned to afrequency slightly above circuit is tuned to a value being an equalamount below the I. F. Each of the resonance circuits feeds a rectifierG11 and Glz, respectively, the rectified currents being caused totraverse a resistance W in opposite directions. If, then, the ensuing I.F. corresponds to the prescribed or rated value, the effects of thecurrent fiowing through the resistance W cancel one another. But if theI. F., owing to inaccurate tuning of the oscillation circuit LC of theheterodyne differs from the rated value, then one of the two rectifiercurrents will predominate over the other, and a drop of potential willbe occasioned across the resistance W in a definite direction. If thisfall of voltage is impressed upon the grid circuit of the tubefurnishing the negative resistance, the operating point will be causedto shift either in the direction towards p or towards q (Fig. 4), withthe consequence that either a positive or a negative capacity capacityC, and thus either an increase or a decrease in the beat frequency isobtained.

Finally, the utilization of the reduction of capacity occasioned byinfluence actions shall be discussed as applied to amplifier circuitorganizations. In the operation of resistance-coupled amplifiersdesigned to amplify a broad band of frequencies, the invariable demandis that the gain should drop as little as possible for the highfrequencies. The cause of such decrease in gain the I. F., while theother.

is connected in parallel to the with increase of frequency resides inthe capacitances of the tube and associated circuits which inevitablyarise in parallel relation to the outer resistance. Now, according tothe invention the suggestion is made to use in one, in several, or inall stages of a resistance-coupled amplifier, especially an amplifieradapted to handling broad-frequency bands, tubes which are operatedunder such conditions that by the influence action of the electrons theinput capacity of the succeeding stage being in parallel relation to anouter resistance is diminished. As a result the capacitive shunt of theouter resistance is diminished and thereby the frequency response curveimproved; or else, for an unchanged frequencyresponse curve a higheroutside resistance may be chosen and thus a higher gain be obtained. Forthe said purpose suitable tubes are used in which the capacity decreaseis as marked as possible. The dynamic input capacity of each tube shouldsuitably be less than '70 per cent of the cold capacity. Most favorableis the use of tubes the input capacity of which is negative, as aconsequence of the above described influence action seeing that thisinput capacitance has connected in parallel to itself in addition theline capacities and the output capacity of the preceding or input tube.To be sure, from the viewpoint of the frequency-response curve of thegain,

it is important only that the sum total of these capacities should be aslowas possible or even slightly negative.

In the light of all previous experience this eifect is obtainable,better than with one-grid tubes (triodes), with tubes in which a controlaction upon the electrons is exercised after they have been acceleratedto a positive potential impressed upon an auxiliary grid (distributioncontrol). In

.ing tube or input tube.

other words, according to this invention, types of tube such asspace-charge grid tubes, hexodes, heptodes, octodes, etc., areparticularly well suited, the alternating potential to be amplifiedbeing impressed upon the control grid nearer the plate. What isfavorable from the viewpoint of utilization of the effect is the factthat the capacity variation due to influence action is fairlyindependent of frequency.

In Fig. 6 is shown an exemplified embodiment of such a circuitarrangement. Included in the plate circuit of the input tube R1, uponthe grid of which the potential to be amplified is impressed acrossterminals a, b, is the resistance Ra, and in shunt relation to thelatter is the capacity Cp which exercises an unfavorable effect upon thefrequency response. The succeeding tube R2 is predicated for itsoperation upon the principle of current-distribution control; this tubecontains the four grids Gi-Gi, in addition to the cathode K and theanode A, the amplified potentials being taken off the plate circuitacross the output terminals 0, d. The third grid G3 is impressed withthe output potential of the preced- Grids G2 and G4 are impressed withconstant positive potentials, grid G1 is impressed with a constantnegative or a feebly positive potential. The adjustment of the dischargeconditions occurs according to the above described prescriptions, sothat the input capacitance Ce prevailing between the control grid G3 andthe filament will be less than the cold capacitance between the said twoelectrodes, and preferably negative in value so that the harmfulparallel capacitance Cp is wholly or partly compensated. As pointed outon page 2, the grid bias is sochosen that the operating point of thetube is at a part of the plate currentgrid voltage characteristic whichis concave downwards. The grid bias source Ug supplies the bias to gridG3 through the usual grid leak resistor. It is at the part of thecharacteristic which is concave downwards that the exponent n is lessthan unity. The grid G3 is shielded by G4 and G2 from the influence ofvarying potentials of other electrodes. The grids G2 and G4 aremaintained at an unvarying positive potential since no load is insertedin circuit therewith.

In addition to the application of this effect for the purpose ofimproving the frequencyresponse curve of resistance coupled amplifiers,there may be mentioned still another chance for using the same. Thedamping d of a parallel oscillation circuit is given by:

C d r where r is the series loss resistance, C the capacitance, and Lthe inductance of the oscillation circuit. Inasmuch as the damping isproportional to C, reduction of C will imply also a reduction ofdamping, in other Words, this leads to a raise of resonance resistanceand the selectivity of the fiy-whee1 circuit. Now, according to theinvention the effect of capacity reduction due to influence effects isto be utilized for the regeneration (deattenuation) of oscillatorycircuits. From a technical angle, such regeneration of oscillationcircuits is useful and desirable in a great many circuit organizationsused in radio-frequency work. As concerns the choice of the tubes and ofthe operating conditions which will be most suited to obtain the saideffect of regeneration, reference is here made to what has been pointedout above, for the preceding remarks and explanation apply also to thiscase.

While we have indicated and described various systems for carrying ourinvention into efiect, it will be apparent to one skilled in the artthat our invention is by no means limited to the particularorganizations shown and described, but that many modifications may bemade without departing from the scope of our invention as set forth inthe appended claims.

What is claimed is:

1. In an electronic network adapted to produce a negative capacityeffect, a positive mutual conductance tube having a cathode, a plate andat least two control grids arranged in succession therebetween, the oneof said grids adjacent the cathode being maintained at an invariablepositive potential, means maintaining said plate at a positivepotential, means establishing the second one of said grids at a negativepotential with respect to the cathode, and the magnitude of the saidnegative potential being so chosen that the operating point of the tubeis on that portion of the plate current-second grid potentialcharacteristic which is concave downwards, said negative capacity efiectexisting between said second grid and said cathode.

2. In an electronic network adapted to produce a negative capacityeifect, a positive mutual conductance tube having a cathode, a plate andat least two control grids arranged in succession therebetween, the oneof said grids adjacent the cathode being maintained at an invariablepositive potential, means maintaining said plate at a positivepotential, means establishing the second one of said grids at a negativepotential with respect to the cathode, and the magnitude of the saidnegative potential being so chosen that the operating point of the tubeis on that portion of the plate current-second grid potentialcharacteristic which is concave downwards, said negative capacity efiectexisting between said second grid and said cathode, and a third controlgrid located between the plate and the second grid, said third gridbeing maintained at an invariable positive potential.

HORST ROTHE. WERNER KLEEN.

